Life History

Age, Growth, Reproduction, and Survival

Belugas are long-lived and have a relatively slow reproductive cycle, giving birth to a single calf every two, three, or more years, and devoting considerable time to caring for their young. Although some life history data are available for CI belugas, considerably more data exist for other beluga populations (see Table 1). Most general beluga life history data have been obtained through measurements and samples of animals taken in subsistence harvests, although some information has come from live stranded, dead beach-cast or floating whales, and some from captive belugas. Relatively little life history data are available specifically for CI belugas.

To understand growth, reproduction, and survival rates, investigators must determine the age structure of the population. Age is primarily assessed by counting the number of growth layer groups (GLGs) of teeth in thin longitudinal sections (see Appendix IX.D – CI Beluga Natural History Supplement). Historically, it was believed that belugas might live more than 30 years (Burns and Seaman 1986); however, it is now thought that belugas may live 60 to 70 years or more (Suydam 2009). It is difficult to know the exact age of older belugas because their teeth wear down and some GLGs are lost as animals age; therefore it is likely that ages determined by counting GLGs are underestimates. For teeth of the 102 CI belugas that have been aged using the single GLG method, the oldest CI beluga was estimated to be at least 49 years (Vos 2003; NMFS, unpub. data).

Obtaining information on the age at sexual maturity (ASM) sheds light on reproduction, and increasing or decreasing trends in ASM may help determine ecosystem dynamics. For instance, if ASM decreases over time in females, this might suggest that resources are not limiting population growth. In published literature, estimates of ASM in belugas ranged from 4–14 years for females and 8–15 years for males (Braham 1984; Nowak 1991; Heide-Jørgensen and Teilmann 1994; Suydam 2009; Table 1). While the cause of the wide range of ASM is currently unknown, possible reasons include: 1) animals may mature at different ages among stocks; 2) different methods may have been used to estimate ages; or 3) the definition of ASM may have differed (e.g., age at first ovulation vs. age at first conception vs. age at first birth). Burns and Seaman (1986) estimated the age at first conception for 22 female belugas in northeast Alaska to be between 8 and 13 years (based on 1 GLG per year).

Estimates of the length of gestation for belugas have also varied from 11 to 16 months (Table 1), although data from captive belugas where conception and birth are precisely known indicate a gestation of 15.6 months (Robeck et al. 2005). Calkins (1983) suggested that most calving in Cook Inlet occurs from mid-May to mid-July. Alaska Native hunters have reported calving from April through August (Huntington 2000). More recently, observations of neonates during annual photo-identification surveys of CI belugas conducted from spring to fall in upper Cook Inlet (2005 to 2015) and the Kenai River Delta (2011 to 2013) led McGuire et al. (2016) to conclude the peak calving period for CI belugas is mid-July through mid-October. The lactation period for belugas is known to last at least a year, and likely longer in some cases. This estimate is based on observations of lactating females that are pregnant with a new fetus and with some estimates of weaning not occurring for about two years; thus, the entire reproductive process on average takes three years (Sergeant 1973; Burns and Seaman 1986). Depending on the age and experience of the mother, however, the calving interval may be as short as two years or over three years (Suydam 2009). Many studies suggest a calving interval for belugas of approximately three years, which equates to a pregnancy rate of about 0.33 per year (Kleinenberg et al. 1969; Sergeant 1973; Ognetov 1985; Burns and Seaman 1986; Doidge 1990b; Heide-Jørgensen et al.1994). This would indicate that approximately one-third of mature females would be newly pregnant in any given year. However, belugas in Hudson Bay, Canada, and Point Lay, Alaska, had greater pregnancy rates of 0.47 (Hudson Bay; Sergeant 1973; Doidge 1990a) and 0.41 (Point Lay; Suydam 2009) indicating calving intervals shorter than three years. Several studies have also suggested a decrease in the pregnancy rate (based on studies of the ovaries) as a female beluga ages, particularly after 40 years (GLGs) (Brodie 1982; Heide-Jørgensen and Teilmann 1994; Suydam 2009). Kleinberg et al. (1969; as presented in Brodie 1971) arbitrarily estimated age at senescence to be around 42–43 years (GLGs). However, this does not mean that older female belugas are not capable of reproducing past this age; a 68 year old female beluga in the St. Lawrence Estuary population in Canada showed signs of recent reproductive activity (McAlpine et al. 1999 as cited in DFO 2012).

In 2005 NMFS began August calf surveys of Cook Inlet. Calving indices were estimated for the period from 2006 to 2012, and indicated that more calves were born by the time the August surveys were conducted in 2006 (12%) than in subsequent years 2007 to 2012, when the average rate was 1.9% (Hobbs et al. 2015a). These calving indices have several potential biases; accordingly, they should be used for trend analysis only, and not for absolute estimates of calf production. They indicate considerable variability from year to year so that a much longer time series is required to determine an average. A similar observation has been made in the SLE beluga population where annual calf production appears to be cyclical (R. Michaud, Group for Research and Education of Marine Mammals, unpub. data). Also, it was assumed, based on mid- July beluga calf sightings in Cook Inlet during aerial surveys in the 1970s, and May calving reported by Alaska Native hunters for the Susitna area, that most calves were born by August. However, during annual photo-identification studies (2005 to 2015), McGuire et al. (2016) reported observations of neonate belugas in upper Cook Inlet extending from July through mid- October.

Table 1. Review of female beluga life history parameters found in the published literature.

>2 years (based on the assumption that females produce 10 calves within a 14-15 year active breeding period)

6e

2-3 years

15

a Sampling occurred in June, a time when most Alaskan belugas are born. It is possible non–pregnant 8–9 GLGs belugas would have conceived before their 10–11 GLGs birth date.b Found differences in maximum age based on sampling technique. Life span of netted whales tended to be lower (40 GLGs at Whale Cove) than those selected and harpooned (50 GLGs at Churchill, 53 GLGs at Mackenzie Delta). Similar results were reported by Brodie (1971) for whales netted in Cumberland Sound (40 GLGs).c In 7 of the 29 pregnant females examined from Whale Cove, lactation was still occurring and for some analyses a 2 year calving cycle was assumed for 25% of the adult female population (p. 1084). Sergeant (1973) concluded “overlap of pregnancy and previous lactation is infrequent so that calving occurs about once in 3 years.”d For some female belugas. This was a tentative conclusion based on high conception rates noted in some females between the ages of 12–13 GLGs and 44–45 GLGs.eBraham (1984) based this assumption on data from Brodie (1971) and Sergeant (1973) that age at first pregnancy is 6 years (12 GLGs) and last pregnancy is about 21 years (42 GLGs) resulting in a 14–15 year breeding period, which would allow only 6 calves rather than the 10 calves predicted by the authors if a female’s reproductive cycle is 3 years. However, this calculation was based on 2 GLGs = 1 year, using 42–12 = a 30- year breeding period and a 3-year reproductive cycle would produce 10 calves.

Based on gestation and timing of birthing, mating is believed to occur sometime between late winter and early spring; however, there is little documentation on the mating behavior of belugas. A reproductive study of belugas in captivity reported that all conception (n = 13) occurred from February to June, with 80.6% of the conceptions occurring from March to May (Robeck et al. 2005). Suydam (2009) stated it was unlikely the eastern Chukchi Sea belugas became pregnant after late June since they did not observe fetuses of a length indicative of an August or September birth date.

Survival data for CI belugas consist of annual summaries of beach-cast and floating carcasses reported to the NMFS AKR, and consequently represent a minimum estimate of mortality for the CI beluga. From 1999 to 2005, when the population size averaged approximately 350 animals and a limited harvest of CI belugas occurred, an average of 12 mortalities were reported each year (Vos and Shelden 2005). This provided an estimated annual survival probability for CI belugas of 0.97 per year. From the literature, survival probabilities for belugas have been estimated as low as 0.84 per year but most were above 0.90 per year. The lactation period is known to last longer than one year, so calf survival closely relates to survival of the mother during the first year following birth. While survival rates and age at maturity have been estimated for males, these estimates did not significantly differ from those for females.

Data are not available for the CI beluga population to precisely determine the generation time, however, when we consider available information regarding the age at first reproduction and age at senescence for belugas, we estimate a generation time for belugas of approximately 25 years. The International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species 10 estimated the generation time for belugas in Cook Inlet to be 16 years based on the information provided by Burns and Seaman (1986), which considered a year to be represented by two GLGs, rather than the currently recognized one GLG/year. Thus, 16 years may underrepresent the actual generation time for belugas. The generation time of between 26 to 30 years has been proposed for belugas in the St. Lawrence Estuary (David Lee, Committee on the Status of Wildlife in Canada [COSEWIC] Member, pers. comm. to R. Hobbs, AFSC MML, October 2014). Therefore, we determine our estimate of generation time of 25 years to be reasonable.